Twenty-nine NU-5 series Arco atomic powered pacemakers continue to function without mechanical difficulties in dogs. Five humans have received Arco NU-5 series pacemakers; three continue to function well. The two Arco pacemakers removed from patients (secondary skin erosion) demonstrate the undesirable design configuration of this pacemaker. One of these pacemakers also showed a predictable decline in pacing rate of 0.1 beat per week, with the rate finally stabilizing at 70. The concept of Plutonium 238 as a power source for this cardiac device continues to be an excellent one. No radiation hazards have resulted from the use of this atomic powered cardiac pacemaker.